1. Field of the Invention
The present invention relates to an image capturing device, a color measuring device, a color measuring system, an image forming apparatus, and a color measuring method, and in particular to an image capturing device, an image capturing system, a color measuring device, a color measuring system, and an image forming apparatus that stably capture a subject and a reference chart.
2. Description of the Related Art
Along with the improvement in image quality, image forming apparatuses, such as color ink-jet image forming apparatuses and color electrophotography image forming apparatuses, are used for offset printing of an advertising medium, a brochure, and other mediums that are printed in a relatively small number of sets but require high image quality.
In offset printing desired to achieve high image quality, colors of printed materials desired by a customer may possibly be different from colors of printout results actually printed out by an image forming apparatus.
Typically, a customer checks the colors of a printed material on a display to order printing. Image forming apparatuses, however, have color reproduction characteristics unique to each model and may possibly produce printing results different from the colors checked on the display.
To address this, conventionally used is a technology for reproducing colors by using a L*a*b* color space and an xyz color space, that is, a color space independent of a device, such as a display and an image forming apparatus, for example.
Image forming apparatuses control the amount of color materials and other factors to output specified colors. Ink-jet image forming apparatuses, for example, perform arithmetic control on the amount of ejected ink and a print pattern to control the amount of ink ejected from an ink head, thereby controlling output colors.
Electrophotography image forming apparatuses, for example, control the amount of toner adhering to a photosensitive element and the light intensity of a laser beam, thereby controlling output colors.
However, the amount of color materials, that is, the amount of ejected ink in an ink-jet image forming apparatus, for example, fluctuates depending on the state of a head nozzle, fluctuation in viscosity of the ink, and fluctuation in an ejection drive element (e.g., a piezoelectric element). As a result, fluctuation occurs in color reproducibility. Furthermore, the amount of ejected ink in ink-jet image forming apparatuses changes over time in one image forming apparatus or varies among image forming apparatuses. As a result, fluctuation occurs in color reproduction of an image over time or among image forming apparatuses.
To address this, image forming apparatuses conventionally perform color adjustment to suppress output fluctuation caused by characteristics unique to each device and to improve output reproducibility corresponding to input. The color adjustment is performed as follows. An image forming apparatus actually outputs an image of a color patch in a reference color (a reference color-patch image), and a color measuring device performs color measurement on the reference color-patch image. Subsequently, a color conversion parameter is generated based on difference between the colorimetric value of the reference color-patch image measured by the color measuring device and the color specification value of a reference color corresponding thereto in a standard color space. The color conversion parameter is then set to the image forming apparatus. To output an image corresponding to input image data, the image forming apparatus performs color conversion on the input image data based on the color conversion parameter thus set and records and outputs an image based on the image data on which the color conversion is performed. Thus, the image forming apparatus suppresses output fluctuation caused by characteristics unique to each device and outputs an image with high color reproducibility.
In the conventional color adjustment, spectrophotometers are widely used as the color measuring device that measures the reference color-patch image. Because spectrophotometers can obtain spectral reflectivity in each wavelength, they can perform highly accurate color measurement. Spectrophotometers, however, are expensive devices equipped with a number of sensors. Thus, it is required to perform highly accurate color measurement with a cheaper device.
Japanese Patent Application Laid-open No. 5-223642 discloses a color measuring device including a reference color measuring unit, a color image input unit, an image extracting unit, and an arithmetic unit. The reference color measuring unit performs color measurement on a reference color chart in advance so as to acquire a color reference value in an RGB data format. The color image input unit acquires RGB data by capturing a subject including the reference color chart and an object on which color measurement is to be performed simultaneously or separately. The image extracting unit extracts RGB data of the reference color chart and RGB data of the object on which color measurement is to be performed from the RGB data acquired by the color image input unit. The arithmetic unit calculates difference between the RGB data of the reference color chart acquired by the image extracting unit and the RGB data of the reference color chart acquired by the reference color measuring unit and corrects at least the RGB data of the object on which color measurement is to be performed using the difference. In the conventional technology, the reference color chart serving as a target for comparison with the subject is placed near the subject to be a target for color measurement. A color video camera serving as the color image input unit then captures the subject and the reference color chart simultaneously. Subsequently, the color measuring device corrects the RGB data of the subject using the RGB data of the reference color chart acquired by the image capturing to transform the RGB data of the subject into a color specification value in a standard color space.
In the conventional technology disclosed in Japanese Patent Application Laid-open No. 5-223642, however, it is difficult to keep the positional relationship constant between the reference color chart to be captured as the target for comparison with the subject and the color image input unit in each image capturing. As a result, the image capturing conditions vary in each image capturing, thereby making it difficult to perform stable image capturing.
Therefore, there is a need for an image capturing device, a color measuring device, a color measuring system, an image forming apparatus, and a color measuring method that are capable of capturing a subject and a reference chart in a stable positional relationship.